1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus, particularly to a solid-state image pickup apparatus that captures a plurality of items of color data by means of a plurality of pixel arrays.
2. Description of the Related Art
Modern solid-state image pickup devices, typically fabricated using complementary metal-oxide semiconductor (CMOS) technology, have various applications, such as in digital still cameras, camcorders, surveillance cameras, etc. Of these solid-state image pickup devices, the single-chip type, which captures a plurality of items of color data by means of a single pixel-array, is the most common.
Continuing demand to further increase the pixel count, reduce the size of an image pickup device optical system, etc., has led to a reduction in pixel size. Pixel size reduction decreases the area of an image pickup area and hence the area of the image pickup device optical system, and reduces focal length, thus allowing a reduction in the height of the image pickup device optical system. The pixel sizes of CMOS sensors used in current digital cameras, etc., range from about 1.4 to 2.8 μm. However, such very small pixels cause problems as described below.
First, if the pixel size is reduced, the number of photons able to impinge on a unit pixel decreases in proportion to the area of the unit pixel. Accordingly, the video signal-to-noise ratio decreases. If the signal-to-noise ratio cannot be maintained, the quality of reproduced images on a display screen will be degraded.
Second, if the pixel size is reduced, crosstalk between adjacent pixels increases. Consequently, pixels assigned for sensitivity to corresponding specific wavelength areas may become sensitive to other wavelengths as well. This may result in color mixing and significant degradation in color reproducibility on a display screen.
Therefore, in order to maintain a high signal-to-noise ratio despite a pixel size reduction and minimize sensitivity degradation, any decrease in the signal-to-noise ratio has to be prevented. In addition, in order to prevent degradation in color reproducibility because of pixel size reduction, and to minimize color mixing, some ingenuity is required. Since an image pickup device has these characteristic drawbacks, it is difficult to reduce pixel size. Accordingly, the image pickup device optical system size cannot be reduced sufficiently.
In order to reduce the foregoing problems, in particular, in order to decrease the thickness of an image pickup device optical system, technologies have conventionally been proposed as in Jpn. Pat. Appln. KOKAI Publication Nos. 2001-78212 and 2006-246193. In these technologies, a plurality of image pickup devices is divided into a plurality of image pickup areas; optical image formation systems are provided for the corresponding image pickup areas; and spectral filters are provided for the corresponding optical image formation systems so as to receive the incident light rays having spectral distributions that differ among the image pickup areas. In this case, the image pickup areas are disposed in contact with one another. Dividing one large image pickup area into several separate image pickup areas in this way makes it possible to reduce the size of each image pickup area and hence the height of the corresponding optical image formation system in the direction of the optical axis. This realizes a camera module of low height.
However, disposing a plurality of image pickup areas in contact with one another generates the following problems: the size of image circle formed by each optical image formation system is larger than the area of the corresponding image pickup area such that light from the optical image formation system disposed in the image pickup area projects onto the adjacent image pickup area, with the result that this light produces a false signal, resulting in significant degradation of the image quality on the display screen. Therefore, the image pickup apparatuses disclosed in the foregoing publications adopt a structure in which a light shield effect is applied between the optical image formation systems.
However, such a method is insufficient to prevent a false signal from entering adjacent image pickup areas. Equally, light reflection may be caused by a light shield wall, leading to vignetting and hence flare. Accordingly, the quality of a reproduced image may be significantly degraded.